style: Implement style system support for Masonry layout.

This implements support for this CSS Masonry layout proposal:
https://github.com/w3c/csswg-drafts/issues/4650

I've intentionally left out a shorthand (place-tracks?) for now until
we have a draft CSS spec for this.

Differential Revision: https://phabricator.services.mozilla.com/D67061

components/style/values/generics/grid.rs

@@ -786,6 +786,9 @@ pub enum GenericGridTemplateComponent<L, I> {
     /// TODO: Support animations for this after subgrid is addressed in [grid-2] spec.
     #[animation(error)]
     Subgrid(Box<LineNameList>),
+    /// `masonry` value.
+    /// https://github.com/w3c/csswg-drafts/issues/4650
+    Masonry,
 }
 
 pub use self::GenericGridTemplateComponent as GridTemplateComponent;